It depends on the group you are auditioning for. I wouldn't say a suit is neccessary, but you definately want to look nice like you were taking it seriously. I shirt and tie, or even just a nice looking (but not too flashy) button up dress shirt, and slacks or nice pants.
Yeah, It DEFINATELY depends on what you're auditioning for. When I auditioned for all-state I dressed in a dress, but when I auditioned for all-county I wore nice pants and a blouse, and when I auditioned for the county youth orchestra I wore pants and a nicer shirt. I hope I helped some.
Black shirt and pants are always a good bet for auditions,not too sure about a tie though,feels uncomfortable to me when I'm playing.Most importantly,wear something you're comfortable in as you don't want to feel awkward when trying to play your best!Good luck.